Many emissive display devices exist within the market today. Among the displays that are available are thin-film, coated, electro-luminescent displays, such as Organic Light-Emitting Diode (OLED) displays. These displays can be driven using active matrix backplanes, which employ an active circuit, or passive matrix backplanes, which provide common signals to rows and columns of light-emitting elements.
In typical, prior-art OLED displays, it is known that the luminance of the different color emitters, e.g. red, green, and blue OLEDs, increases as current density delivered to the OLED is increased. The transfer function from current density to luminance typically behaves according to a linear function. Therefore, to increase the luminance of the display, one must increase the current delivered to an OLED with a given area. To maintain a color-balanced display, the current must be adjusted differentially to the three OLEDs to maintain the desired ratio of red:green:blue luminance.
Unfortunately, increasing the current density used to drive an OLED, and therefore the luminance, not only increases the power required to drive the OLED but also reduces the lifetime of the OLED. Perhaps of greater importance is not the overall aging but the fact that the aging of the different colors is not the same. Therefore, the luminance of some colors will degrade faster than others. To maintain a well-balanced, full color display, it is important that the relative luminance of the colored materials be maintained throughout the lifetime of the display.
The overall lifetime of a display can decrease through changes in relative color efficiency as well as decreasing luminance output. If one OLED material that produces a particular color of light degrades more rapidly than other materials that produce other colors of light, for example through heavier use, the particular light output from the material will decrease relative to the other colors. This differential color output change will change the color balance of the display, such that images may have a serious color imbalance, which is much more noticeable than a decrease in overall luminance. While this decrease in luminance and light output of the particular color can be compensated for by increasing the brightness of the particular color, such a solution increases the rate of aging and the power usage and exacerbates the change in relative color efficiency in the display. Alternatively, one can reduce the luminance of the more robust colors, but this lowers the overall brightness of the display. To maximize the useful lifetime of the display, it is important to maximize the time that the relative luminance of the color elements can be maintained while minimizing the loss of absolute luminance.
Flat panel displays with unequal areas of light-emitting material have been discussed by Kim et al. in US Patent Application 2002/0014837. The relative size of the red, green, and blue light-emitting elements are adjusted based on the luminous efficiency of the color materials employed in an OLED display. In some display configurations, the available red OLED materials have significantly lower luminous efficiency than the existing green and blue OLED materials. Because of the lower efficiency of existing red OLED materials, if one wishes to maintain sub-pixels of equal size, the power per square area that must be provided to the low luminous efficiency material must be increased to obtain the desired light output. Using this criterion, Kim proposes an OLED display with a larger red-light-emitting area than the green- and blue-light-emitting areas. Thus, the relative power per area can be somewhat equalized across the different colored materials. However, optimizing the display layout suggested by Kim et al., does not necessarily lead one to a design in which the lifetimes of the three materials are optimized.
U.S. Pat. No. 6,366,025 by Yamada discloses an OLED display with unequal light-emitting element areas, wherein the areas of the light-emitting elements are adjusted with the goal of improving the lifetime of the OLED display. Yamada considers the emission efficiency of the material, the chromaticity of each of the emissive materials, and the chromaticity of the target display when attempting to determine the aim light-emissive element areas. However, Yamada fails to discuss other important characteristics of OLED materials that will affect device lifetime, such as the differences in the inherent luminance stability over time of different materials. More importantly, typical manufacturing approaches limit the maximum differences in the areas of the different colored subpixels. As such, this approach alone cannot compensate for all of the differences in emission efficiency of the materials, or for other important factors, such as optical characteristics or differences in the inherent luminance stability of the different materials that are typically used to form the differently colored subpixels.